What distance should my light be from my plants?

The Inverse Square Law​

The Inverse Square Law is a fundamental principle in physics that states that light energy diminishes exponentially as the distance from the source increases. For indoor plant growers, this law emphasizes the importance of positioning the light source as close as possible to the plants to optimize yield and flower density.

The Problem: Heat from High Intensity Discharge Lamps​

High Intensity Discharge (HID) lamps, commonly used for indoor plant cultivation, generate significant amounts of heat. Placing these lamps too close to plants can cause damage. Therefore, finding the Optimum Lamp Height (OLH) is crucial for balancing maximum light intensity with heat management.

Finding the Optimum Lamp Height (OLH)​

To determine the OLH, the lamp should be positioned at a height where it fully illuminates the canopy with direct light while being as close to the plants as possible. To achieve this without causing harm to the plants, consider the following steps:

1. Test your plants' tolerance: Move the light to the OLH and observe the plants' reaction. If they can withstand the heat, there's no need to make further adjustments.
2. Exhaust the light hood: If the plants are affected by the heat, try actively exhausting the light hood with a direct exhaust system. Move the lamp to the OLH again and monitor the plants.
3. Add glass to the lamp hood: If the plants still struggle with the heat, add glass to the actively exhausted lamp hood. While the glass may reflect a small percentage of PAR wattage, it will filter out most of the UV-B light, which is beneficial for cannabis potency.

Choosing the Right Glass and Other Considerations​

Most available glass for air-cooled lamp hoods is tempered glass, which has low impurities. For maximum efficiency, quartz glass allows UV-B transmission but isn't specifically designed for light hoods.

Remember to position the longest side of the reflector parallel to the shortest side of your garden for rectangular gardens. Additionally, regularly inspect and clean your light hood and bulbs, as dust and dirt can reduce reflectivity. Use isopropanol alcohol, glass cleaner, or water and a soft cloth to clean the surfaces.

Temperature Factors and Testing​

Several factors, such as wattage, hood design, and air circulation, affect the temperature radiated from your bulb. A simple method for testing temperature is using the back of your hand. If it's too hot for your hand, it's too hot for your plants. Good ventilation is essential for bringing the light closer to the garden.

Editor's Note: Recommended Typical OLH Distances​

• Fluorescents: proximity
• 400w HPS: 1 foot
• 600w HPS: 1.5 foot
• 1000w HPS: 2 foot